1. Field of the Invention
The present invention relates to a piezoelectric loudspeaker having flat reproduced sound volume characteristics in a wide range of frequencies.
2. Description of the Related Art
Conventional piezoelectric loudspeaker structures are disclosed in, for example, Japanese Laid-Open Utility Model Publication No. 63-81595 and Japanese Laid-Open Patent Publication No. 1-135299.
FIG. 38 illustrates the structure of a conventional piezoelectric loudspeaker disclosed in Japanese Laid-Open Utility Model Publication No. 63-81595. This conventional piezoelectric loudspeaker includes a piezoelectric diaphragm, which is composed of two thin piezoelectric members 31A and 31B electrodes 32A, 32B, 32C attached together, and a kneaded mixture 33 of metal powder and adhesive. Reference numerals 34A and 34B denote electrical input lines. The mixture 33 is adhered to the central portion of the piezoelectric diaphragm so as to damp the resonance peak Q at the resonance points of the piezoelectric loudspeaker, thereby improving the vibration mode characteristics. This technique of attaching visco-elastic member, i.e., the kneaded mixture 33 in a smaller diameter of the central portion of the diaphragm results in a somewhat reduced resonance peak. However, the amount of resonance attenuation obtained is well below a sufficient level. As a result, this conventional piezoelectric loudspeaker cannot attain sufficiently flat reproduced sound volume-frequency characteristics.
FIG. 39 illustrates the structure of a conventional piezoelectric loudspeaker disclosed in Japanese Laid-Open Patent Publication No. 1-135299. This conventional piezoelectric loudspeaker includes: a metal diaphragm 35; a piezoelectric vibrator composed of a thin piezoelectric member 36 attached to one side of the diaphragm 35; a soft damper plate 39 made of foam resin, rubber, or the like with the same outer diameter as that of the metal diaphragm 35 overlaid on the opposite side of the metal diaphragm 35: and a frame 37 for sandwiching the metal diaphragm 35 and the damper plate 39 at their outer peripheries around their entire circumferences for providing stable support for the metal diaphragm 35 and the damper plate 39. Reference numerals 38A and 38B denote electrical input lines. It is possible to impart this piezoelectric loudspeaker with various frequency characteristics by selecting the sandwiching pressure in accordance with the materials composing the metal diaphragm 35 and the damper plate 39. However, in accordance with the above conventional structure, the soft damper plate 39 and the metal diaphragm 35 are sandwiched by the frame 37 at their outer peripheries. Such a soft damper plate 39, extending all the way to the frame 37, may allow the vibratory force induced by the vibrator 36 to be transmitted into the frame 37. As a result, this conventional piezoelectric loudspeaker will not attain large vibration amplitude and hence it will not reproduce sounds at high sound volume.
Moreover, as the input voltage to the aforementioned conventional piezoelectric loudspeakers is increased, an excessive amplitude may occur, especially within the vicinity of the central portion of the piezoelectric vibrator. This may result in the xe2x80x98peelingxe2x80x99 of the piezoelectric member from the metal diaphragm, and even the destruction of the piezoelectrio vibrator.
It is known that the lowest reproducible frequency in the reproduced sound volume characteristics can be lowered as the diameter of the metal diaphragm is increased. However, this also creates a corresponding decrease in the highest reproducible frequency. Thus, it is difficult to attain a wide reproduction band width with the conventional piezoelectric loudspeakers of the type discussed above.
In a further case where a piezoelectric member is affixed on each side of the metal diaphragm, it is necessary to affix lead wires to both piezoelectric loudspeakers, and the lead wires must lead out from above and below the frame. This is aesthetically and electrically unpreferable, and may create problems such as peeling, entanglement, or even severance of the lead wires, resulting in a malfunction.
A piezoelectric loudspeaker according to the present invention includes: a piezoelectric vibrator including a diaphragm and a piezoelectric member provided on at least one face of the diaphragm, the diaphragm being vibrated by the piezoelectric member: a frame for supporting the piezoelectric vibrator: and a visco-elastic member provided on at least one face of the piezoelectric vibrator, wherein the visco-elastic member is disposed in a substantial center of the piezoelectric vibrator, and wherein the visco-elastic member has a bottom face area which accounts for about 11% to about 80% of a bottom face area of the diaphragm.
In one embodiment of the invention, the visco-elastic member includes two or more visco-elastic members stacked on top of each other, and the two or more types of visco-elastic members at least have different materials or different shapes.
In another embodiment of the invention, the visco-elastic member includes first and second visco-elastic members which are provided on opposite sides of the piezoelectric vibrator.
In still another embodiment of the invention, the first and second, visco-elastic members at least have different materials or different shapes.
In still another embodiment of the invention, the visco-elastic member includes two or more visco-elastic members having mutually different values in at least one of specific gravity, Young""s modulus, and internal loss, and the two or more types of visco-elastic members are disposed in a concentric manner.
In still another embodiment of the invention, a rigid member is provided on the visco-elastic member, the rigid member having a specific gravity which is larger than a specific gravity of the visco-elastic member.
In still another embodiment of the invention, the piezoelectric vibrator has at least one aperture, the at least one aperture being at least partially filled by the visco-elastic member.
In still another embodiment of the invention, the frame has a horn-like configuration having an opening, the opening having a gradually increasing cross-sectional area away from the piezoelectric vibrator and toward a final opening at which soundwaves are emitted, and the visco-elastic member has a conical configuration having a gradually decreasing cross-sectional area away from the piezoelectric vibrator and toward the final opening.
In still another embodiment of the invention, the piezoelectric loudspeaker further includes an element provided in a central portion of the visco-elastic member, at least one of specific gravity and elastic modulus of the element being larger than specific gravity and/or elastic modulus of the visco-elastic member.
In still another embodiment of the invention, the visco-elastic member includes notches at least in one portion thereof.
Alternatively, the piezoelectric loudspeaker plurality of includes: a piezoelectric vibrator including a diaphragm and a piezoelectric member provided on at least one face of the diaphragm, the diaphragm being vibrated by the piezoelectric member; a frame for supporting the piezoelectric vibrator; and a support element for supporting the piezoelectric vibrator at a substantial center of the piezoelectric vibrator.
In one embodiment of the invention, the support element includes a conductive portion which is in electrical contact with the piezoelectric vibrator, and an electrical input is applied to the conductive portion.
In another embodiment of the invention, the loudspeaker further includes a visco-elastic member provided on at least one face of the piezoelectric vibrator.
Alternatively, the piezoelectric loudspeaker according to the present invention includes: a piezoelectric vibrator including a diaphragm and a plurality of piezoelectric members provided on at least one face of the diaphragm, the diaphragm being vibrated by the plurality of piezoelectric members; and a frame for supporting the piezoelectric vibrator, wherein different voltages are applied to at least two of the plurality of piezoelectric members.
In one embodiment of the invention, the piezoelectric loudspeaker further includes a visco-elastic member provided on at least one face of the piezoelectric vibrator.
In another embodiment of the invention, at least one of the plurality of piezoelectric members receives an electric input via an electrical resistance.
In still another embodiment of the invention, the plurality of piezoelectric members are defined by at least two split sections of the visco-elastic member provided on at least one face of the piezoelectric vibrator.
In still another embodiment of the invention, the piezoelectric loudspeaker further includes an electrically resistant member for interconnecting at least two of the plurality of piezoelectric members.
In still another embodiment of the invention, the piezoelectric loudspeaker further includes a plate for connecting at least one said visco-elastic member to the frame so as to damp unwanted vibration of the piezoelectric vibrator, wherein an enclosed space is formed by the plate, the frame, and the diaphragm.
In still another embodiment of the invention, the plate has at least through-hole.
In still another embodiment of the invention, the visco-elastic member includes a conductive portion which is in electrical contact with the piezoelectric vibrator, and an electrical input is applied to the conductive portion.
In still another embodiment of the invention, the piezoelectric loudspeaker further includes a lead wire for applying an electric input to the piezoelectric member, wherein the piezoelectric vibrator has at least one through-hole through which the lead wire is coupled to the piezoelectric member.
In still another embodiment of the invention, the piezoelectric loudspeaker further includes a cover for protecting at least one said visco-elastic member and the piezoelectric vibrator.
In still another embodiment of the invention, the piezoelectric loudspeaker further includes a conductive terminal for applying an electrical input to the piezoelectric member, the conductive terminal being provided within the cover.
Alternatively, the piezoelectric loudspeaker according to the present invention includes: a piezoelectric vibrator including a diaphragm and a piezoelectric member provided on at least one face of the diaphragm, the diaphragm being vibrated by the piezoelectric member: a frame for supporting the piezoelectric vibrator; and a visco-elastic member provided on at least one face of the piezoelectric vibrator, wherein the visco-elastic member is disposed in a substantial center of the piezoelectric vibrator, wherein the visco-elastic member has a bottom face area which accounts for about 11% to about 80% of a bottom face area of the diaphragm, and wherein the bottom face area of the visco-elastic member is equal to or greater than the bottom face area of the piezoelectric member, and a diameter of the visco-elastic member is smaller than the inner diameter of the frame.
In one embodiment of the invention, the frame includes a conductive portion which is in electrical contact with the piezoelectric vibrator, and an electrical input is applied to the conductive portion.
Thus, the invention described herein makes possible the advantage of providing a piezoelectric loudspeaker which attains a high reproduced sound volume level, a wide reproduction frequency band width, flat reproduced sound volume-frequency characteristics with a relatively simple structure.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.